In the past, active energy ray-curable inkjet inks are, in comparison with those of solvent type, mounted on signage printers of high-speed printing type due to its quickness of drying, and on flat bed printers for various base materials due to its marked adhesion to the base materials, and its recipes according to the use are under development.
These printers cope with the increase of size, thickness, and density by scanning the head. In recent years, with the advances in head technologies, the head technique allowing ejection of microscopic droplets at a high frequency has been established. Owing to this achievement of the technique, the possibility of replacement of the existing printing system with inkjet printing, which has been inferior in productivity and image quality has increased, accompanied with the merit of digitization. Furthermore, the advent of a single-pass printer, which simultaneously cures the inks ejected from at least two or more inkjet heads, has allowed the cost reduction and enhancement of productivity of the apparatus, and is accelerating the replacement of the existing printing system.
The conversion from the existing printing to inkjet printing is spreading in many fields, because it matches with the recent business model concerning provision of various products in small lots for responding to diversified consumer needs. Especially for label printing, if the application to the volume zone such as food, cosmetics, and pharmaceutical products, is allowed, variable data for each product can be attached to the label, and manufacturers can provide safety and security to consumers at a lower cost.
However, the achievement of this technological innovation required the development of ink having higher sensitivity to active energy rays than the existing ink. High sensitivity contributes to the improvement of printing speed, and is required of inks for the enhancement of productivity.
In addition, improvement of color reproducibility is required for the replacement of prior art offset printing with inkjet printing. Especially in the fields of food, cosmetics, and pharmaceutical products, the sales of products are markedly influenced by the attractiveness of the presentation of the content by the label. Therefore, color reproducibility can be one of important quality factors. However, especially for active energy ray-curable inkjet inks, it is very difficult to satisfy all the demanded properties such as color reproducibility of images, curability, and ejection stability.
If the application amount of the ink composition is increased for achieving a wide range of color reproducibility, the image to be produced is mat and unsuitable for the labels of food, cosmetics, and pharmaceutical products. In addition, if the pigment concentration in the ink composition is increased for achieving a wide range of color reproducibility, the ink composition is thickened, and the inside of the coating film will not be cured, and the residual components in the cured film can invade into food, cosmetics, and pharmaceutical products to cause problems.
Especially in an active energy ray-curable inkjet ink, the ink materials contain odorous matter and harmful components to human body. If these raw material components of the ink remain in the cured film as residue and it invade into the content such as food, cosmetics, and pharmaceutical products, serious problems can occur. In order to prevent invasion of the residue into the content, it is most effective to reduce the residual component amount in the cured film. In other words, a wide range of color reproducibility and reduction of residual components in the cured film are important factors for the use of an active energy ray-curable inkjet ink for labels for food, cosmetics, and pharmaceutical products.
The residual component amount in the cured film can be grasped by immersing the cured film in a highly soluble solvent such as methyl ethyl ketone, and determining the amount of components eluted in the solvent. The use of this criterion allows more accurate measurement of residual component amount in the cured film than the existing measurement method described in Literature 2 wherein the cured film is warmed, and the amount of volatilized components is determined. Therefore, safety is guaranteed and usable for the pharmaceutical products and other applications wherein the invasion of ink components into the product is more strictly limited.
For these problems, in the past, discharge of residual components from the cured film to the outside is prevented by the use of high molecular weight compounds without the use of low molecular weight compounds. However, these high molecular weight compounds increase the ink viscosity, and cannot be abundantly used in inkjet inks required to have a low viscosity for ejection. Furthermore, it was found that high molecular weight compounds have low reactivity, so that remain in the cured film in the form of unreacted components, and can be eluted by methyl ethyl ketone.
Literature 1 suggests a new initiator useful for food applications. However, when an ink containing a pigment 1% by weight or more is prepared by this method, curing is insufficient, and the residual component ratios in the cured film were found to be high; 0.5% or more for the monomers, and more than 6% for the initiator when eluted with methyl ethyl ketone. Therefore, this method cannot be used for labels for food and pharmaceutical products.
Literature 2 suggests the method of providing an ink for packaging toys and food. However, this method requires nitrogen charging during ultraviolet ray curing, which can result in a very high running cost. In addition, the conveyor speed is 10 m/min, and the productivity is very low. Furthermore, if an ink containing a pigment in the ratio of 1% by weight or more is prepared by this method, ejection stability is poor, and high-definition images cannot be made by high-speed continuous printing. In addition, curing is insufficient, and the residual component ratios in the cured film when eluted with methyl ethyl ketone were found to be 0.5% or more for the monomers, and more than 6% for the initiator. Therefore, this method cannot be used for labels for food and pharmaceutical products supporting high-speed printing without any change.
Literature 3 suggests a method of providing an ink containing 2-(2-vinyloxyethoxy)ethyl acrylate as an ink having good curability. However, under this method, the reaction from the initiator to the monomer is hard to proceed, so that curing of the monomer is insufficient. In the ink containing a pigment in the ratio of 1% by weight or more, the residual component ratios in the cured film when eluted with methyl ethyl ketone were found to be 0.5% or more for the monomers, and more than 6% for the initiator.
Literature 4 discloses an active energy ray curable composition having high storage stability while maintaining curability. In addition, Literature 5 discloses an active energy ray-curable inkjet ink set, the ink set giving high quality images especially with a single-pass printer, and having high quality stability after storage by adjusting the blending quantity of the surface tension regulator. Literature 6 discloses an inkjet recording method which is superior in curability, ejection stability, and suppression of temperature rise in the recording apparatus after continuous printing. Either Literatures disclose an active energy ray-curable ink composition using 2-(2-vinyloxyethoxy)ethyl acrylate and dipropylene glycol diacrylate as monomers, but curing is insufficient, and the residual component ratios when eluted with methyl ethyl ketone in the cured film are 0.5% or more for the monomers, and more than 6% for the initiator. Therefore, these inks are not sufficiently safe for the labels for food, cosmetics, and pharmaceutical products, and cannot provide high quality images by high-speed printing.